A phase shift mask (PSM) may include alternating clear regions (which may be designated as zero (0) and pi (π) regions) with different step heights. The different step heights cause the light to be phase-shifted 180° between the two regions. As a consequence, the light from the clear zero region interferes destructively with the light diffracted from the adjacent pi region, causing an unexposed (dark) area on the photoresist layer.
“Phase conflict” is a potential problem in PSM technology. Phase conflict arises from unintentionally joining two regions that transmit light with opposite phases. Destructive interference of the light from these two regions may create artificial features on the wafer (e.g., unwanted lines).
Phase conflict may be avoided by using a double exposure/two mask method. In a first exposure, a PSM is used to image a pattern, which may include undesired features caused by phase conflict. In a second exposure, a binary (e.g., chrome-on-glass) “trim” mask is used to clear the undesired features. However, the addition of the second (trim mask) exposure may introduce overlay errors, reduce flexibility of physical design, and increase cycle time, labor, and production costs. These problems may worsen as the process margins of photolithography decrease and the pattern densities of integrated circuits increase.